Transformers by nature are wide band devices
regarding stray signal coupling. Where a transformer is required to operate in electrically noisy environments, a conducting shielding layer can be interposed between the primary and secondary windings (or between individual secondaries) to minimize the capacitance between them. This can reduce (or even eliminate) some types of common mode noise, but its effectiveness depends on the noise characteristic as well as the transformer's overall surface area.
This type of shield is sometimes required to satisfy certain safety regulations and circuit configurations. Capacitive screens add layers and cost to a toroid's build, because if they are required, a larger core may have to be specified with a large enough inner diameter to complete the windings. This type of shielding should supplement and not replace the usual line filters and suppressor networks that may be required for circuit operation or EMC compliance.
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